1. Field of the Invention
The present invention relates to an RF module used for propagating a signal in a high frequency band of microwaves, millimeter waves, or the like and a mode converting structure and method for converting a mode between different waveguides.
2. Description of the Related Art
Conventionally, as transmission lines for transmitting a high frequency signal in a microwave band, a millimeter wave band, and the like, a strip line, a microstrip line, a coaxial line, a waveguide, a dielectric waveguide, and the like are known. Each of them is also known as a component of a resonator and a filter for high frequency signal. An example of a module formed by using any of the components for high frequency is an MMIC (Monolithic Microwave IC). Hereinbelow, a transmission line for high frequency, and a microstrip line, a waveguide, or the like each serving as a component of a filter or the like will be generically called waveguides.
Propagation modes of electromagnetic waves in a waveguide will now be described. FIGS. 19A and 19B show an electric field distribution and a magnetic field distribution, respectively, in a state called a TE mode (TE10 mode) in a rectangular waveguide. The positions of sections S1, S2, S3, S4 and S5 in FIG. 19A and those in FIG. 19B correspond to each other. FIG. 20 shows an electromagnetic distribution in the section S1. As shown in FIG. 19A. FIG. 19B and FIG. 20, a state in which electric field components exist only in the section direction, and electric field components do not exist in an electromagnetic wave travel direction (waveguide axial direction) Z is called the “TE mode”.
FIGS. 21A and 21B show electromagnetic field distributions in a state called a TM mode (TM11 mode). FIG. 21A shows an electromagnetic field distribution in an XY section orthogonal to the waveguide axial direction Z, and FIG. 21B shows an electromagnetic field distribution in a YZ section of a side face. As shown in FIG. 21A and FIG. 21B, a state in which magnetic field components exist only in the section direction and no magnetic field components exist in the electromagnetic wave travel direction Z is called the “TM mode”.
In each of the modes, a plane parallel to an electric field E is called an “E plane” and an plane parallel to a magnetic field H is called an “H plane”. In the examples of the TE mode of FIGS. 19A and 19B, a plane parallel to the XY plane is the E plane (FIG. 19A), and a plane parallel to the XZ plane is the H plane (FIG. 19B).
In a microstrip line, a coaxial line, or the like shown in FIGS. 22A and 22B, a state called a TEM mode exists. The microstrip line is obtained by, as shown in FIG. 22A, disposing a ground (earth) conductor 101 and a line pattern 103 made of a conductor having a line shape so as to face each other while sandwiching a dielectric 102. The coaxial line is obtained by, as shown in FIG. 22B, surrounding a central conductor 111 by a cylindrical ground conductor 112.
FIGS. 23A and 23B show electromagnetic field distributions in the TEM mode in the microstrip line and the coaxial line, respectively. A state in which, as shown in FIG. 23A and FIG. 23B, both of the electric field components and the magnetic field components exist only in sections and do not exist in the electromagnetic wave travel direction Z is called a “TEM mode”.
In an RF module having a plurality of waveguides, a structure for mutually coupling the waveguides is necessary. In particular, in the case of coupling waveguides of different modes, a structure for performing mode conversion among the waveguides is required.
Conventionally, an example of known structures of connecting a microstrip line and a waveguide is that, as shown in FIG. 24, a ridge 121 is provided in the center of the waveguide. The line pattern 103 of the microstrip line is inserted in a portion where the ridge 121 is provided. In this case, on assumption that the microstrip line is in the TEM mode and the ridge waveguide is in the TE mode, the electric field distribution in the microstrip line is as show in FIG. 25A, and that in the ridge 121 is as shown in FIG. 25B. In a connection portion, by combining both of the electric field distributions, mode conversion is performed between the microstrip line and the ridge waveguide.
Recently, there is a known structure in which a dielectric waveguide line is formed by a stacking technique in a wiring board of a multilayer structure. The structure has a plurality of ground conductors stacked while sandwiching dielectrics and through holes of which inner faces are metalized to make the ground conductors conductive, and electromagnetic waves are propagated in a region surrounded by the ground conductors and the through holes. A structure in which the waveguide having the multilayer structure is connected to a microstrip line is disclosed in, for example, Japanese Unexamined Patent Publication No. 2000-216605. The structure disclosed in this publication is basically similar to the structure using a ridge waveguide. In a center portion of the waveguide, a ridge is falsely formed in a step shape by using the through hole.
Another example of the structure of connecting waveguides of different kinds is that an input/output terminal electrode is provided in an end portion of a base of a dielectric resonator, and the input/output terminal electrode is connected to a line pattern on a printed board (Japanese Unexamined Patent Publication No. 2002-135003).
Conventionally, some structures of connecting different waveguides are known as described above. On the other hand, the waveguide having the multilayer structure is a relatively new technique, and the structure of connecting different waveguides has not been developed sufficiently. In particular, in the case of connecting a waveguide in the TEM mode and a waveguide having the multilayer structure, the converting structure for properly converting the mode among the waveguides has room for improvement.